The invention relates generally to nanostructured ferritic alloys and articles made of such alloys. More particularly, the invention relates to an article having a nanostructured ferritic alloy surface with good corrosion resistance, and methods of forming the article.
Material selection is especially important in equipment components used in sour and acid environments commonly associated with oil and gas extraction installations. Sour gas wells may contain carbon dioxide, chlorides, hydrogen sulfides, and free sulfur, and may operate at temperatures up to 400° C. This type of corrosive environment requires carefully designed alloys to enable components to maintain their structural integrity over their service life.
Conventional corrosion resistant steels include ferritic, austenitic, and ferritic/austenitic duplex steels. In general, ferritic steels have improved stress corrosion cracking resistance in chloride-containing environments, but the strength is relatively low. Austenitic and duplex steels have good corrosion resistance, low to intermediate strength, but inferior stress corrosion cracking resistance.
Nickel-based super alloys have high strength, corrosion resistance, and stress-corrosion cracking resistance. Ni-based super alloys generally include nickel (Ni), as well as other elements such as iron (Fe), chromium (Cr), molybdenum (Mo), tungsten (W), and copper (Cu). Nickel provides resistance to aqueous chloride stress corrosion cracking and provides resistance to alkalis, while iron is generally added to reduce the use of nickel, where appropriate. Molybdenum and tungsten are beneficial for pitting corrosion resistance and provides general corrosion resistance in reducing acids. Chromium improves general corrosion resistance in oxidizing acidic media. Copper is found to be beneficial for general corrosion resistance in non-oxidizing corrosion environments. Relative concentrations of Ni—Fe—Cr—Mo—Cu, along with alloy processing and service history of the component, in part determine overall corrosion resistance in oil and gas applications. Because higher nickel content increases the cost of raw materials, there is a need for alloys with lower nickel content than typical superalloys but having mechanical strength and corrosion resistance in sour and acid environment superior to conventional steels.